1. Field of the Invention
The present invention relates to a fellow soldering apparatus for soldering electronic parts to a circuit substrate by blowing various hot gases of a desired density and temperature. More particularly, the present invention relates to a fellow soldering apparatus capable of economically soldering by reducing the amount of consumed gas supplied to the apparatus, in order to maintain the gas density in the apparatus. Further, the present invention relates to a fellow soldering apparatus which blows a hot gas when a processing object is transferred on a conveyer, and which responds quickly to changes in the density of the gas which change occurs from changes in the gas flow direction, making it possible to reduce variations in the gas density and to obtain satisfactory solderability.
2. Description of the Related Art
In recent years, the circuit density of mounting boards has become increased, and there has been progress in the surface mounting thereof. When electronic parts are soldered to a circuit substrate, the fellow method has come to be widely used from the point of view of soldering reliability and productivity. In solder paste used for soldering, solder particles are formed to have a very fine size and no solid matter in the flux. When solder paste is formed to have this state, and if a circuit substrate is soldered by a conventional reflow method employing air, solder balls and wet defects occur frequently due to oxidation of solder particles and shortage of an activator.
Therefore, a reflow method employing an inert gas such as nitrogen in place of air has come to be used. A typical, known reflow apparatus which uses the above method, is disclosed in, for example, Japanese Patent Laid-Open No. 64-71571.
FIG. 1 is a schematic view illustrating the construction of a conventional reflow soldering apparatus. As shown in FIG. 1, a conventional reflow soldering apparatus comprises three preheating area (preheating chambers) blocks, two main heating area (reflow chambers) blocks, and one cooling area (cooling chamber) block.
Although the preheating chamber and the reflow chamber are connected to each other by an unillustrated portion for transferring a processing object, the chambers are divided by a partition (not shown). In the preheating chamber and the reflow chamber, a gas blowing type infrared heater 140 is used as a heater, disposed in pairs above and below an unillustrated processing object transfer passage (conveyer). A nozzle capable of freely adjusting the direction of gas to be blown is disposed in pairs in upper and lower positions between the infrared heaters 140. The gas, acting as a heat medium, is supplied and blown from a line 146 to the infrared heaters 140, and gas from a line 147 is sent out to nozzles 141 between the infrared heaters 140, and jetted out to an interior of a furnace. The gas blown out from the infrared heaters 140 is collected through a line 148 and then supplied to a cooling nozzle 142 through a blower 144 and a heat exchanger 145.
The processing object is transferred by the conveyer from left to right as indicated by the arrow X in the figure. It is preheated to a desired temperature by three pairs of infrared heaters 140 in the preheating chamber, then heated to the temperature at which solder is melted by two pairs of infrared heaters 140 in the reflow chamber, and cooled by the cooling nozzles 142 and cooling fans 143 in the cooling chamber, thus completing the soldering.
However, the above-described prior art has the following problems. There is a conveyer which passes through the preheating chamber, the reflow chamber and the cooling chamber in order to freely convey a processing object in or out of the apparatus, and the space above and below the conveyer is communicated with the atmospheric air. Therefore, since the gas, which is a heat medium, passes through this communicated portion and flows outside the apparatus, the operational cost of the apparatus increases because expensive gas is consumed, making it less economical. If the amount of supplied gas is decreased, the density of gas varies, oxidation of the solder progresses and satisfactory solderability cannot be obtained.
When the processing object is transferred through the preheating or reflow chamber, gas blown from the infrared heaters 140 and the nozzles 141 strikes the processing object and changes from a vertical movement to a horizontal movement. When this change of direction occurs in the boundary between the preheating chamber and the reflow chamber, or near the boundary between the reflow chamber and the cooling chamber, gas is made to flow out or flow in between the chambers, and the gas density in each chamber varies. For example, when the processing object is positioned near the entrance to the reflow chamber, gas is caused to flow out from the reflow chamber to the preheating chamber. Therefore, the gas pressure in the reflow chamber decreases slightly, and a cool gas having a high oxygen density flows in from the cooling chamber according to the extent of such a decrease. When the density of a nitrogen gas decreases in the reflow chamber, solder is oxidized, or the temperature of the gas decreases, making it impossible to obtain satisfactory soldering.
As a countermeasure for the above, a gas sensor may be disposed in each chamber in order to detect variations in the gas density, and feedback control may be performed so that the shortage may be compensated for in response to the variations. However, at present, no gas sensor which quickly detects slight variations in gas density so as to perform feedback control is available. The gas density may vary greatly by the time the shortage is compensated for, and oxidation of the solder progresses. Therefore, satisfactory soldering cannot be obtained.